In general, when an electrolytic solution comprising free, movable positively and negatively charged ions allowing the solution to conduct electric current (e.g. salt water) is brought into contact with two different metallic substances electrically connected to each other, the less noble metallic substance will corrode more than the more noble metallic substance (the ‘degree of nobility’ being e.g. defined by the activity or Galvanic series listing metals in descending order of the potential they develop in a given electrolyte against a standard reference electrode). The degree of ‘nobility’ of a metal as given by the activity series is based on how strongly its atoms are bound to the surface of the material (the stronger the bond, the more noble the material). A relatively less noble metallic substance X will relatively more easily (than the more noble metallic substance Y) give up atoms to the electrolytic solution in the form of positively charged ions (X+n) and thereby relatively more easily corrode. The rate of corrosion depends on a range of parameters, including the potential difference in the activity series between the two metallic substances in question, the kind of and concentration of ions in the electrolytic solution, the area of the exposed parts, the temperature, etc. The less noble metal is said to constitute the anode and the more noble metal the cathode in an electrochemical corrosion process. This process is termed galvanic corrosion.
In case of the presence of two (e.g. identical) metallic substances in an electrolytic solution with a voltage difference applied between the two metallic substances (e.g. electric terminals or conductors on a substrate driven by different polarities of a battery or just having different potentials), the metallic substance having the more positive potential (the anode) will corrode the faster.
A hearing aid or other communication or listening device having a part located at or in the ear of a person is subject to a harsh environment with heat, moisture, sweat, etc. (the latter constituting an electrolytic solution comprising Na+ and Cl− ions). Typically, the enclosure of the electronic components and the battery is not absolutely tight (although attempts are made to contrary). This leads to corrosion inside the hearing aid, if e.g. sweat is allowed to enter the device. The corrosion will occur at the locations where the electrolytic solution enter, e.g. ‘guided’ by capillary effects, e.g. by micro volumes or channels. Such micro volumes or channels can e.g. be present between a battery compartment or an external activation element (e.g. a push button) and an enclosure hosting a printed circuit board. The presence of an electrolytic solution combined with the relatively high energy (voltage difference in the order of one or more Volts) in the battery, makes it almost certain that—at some stage of the lifetime of the device—there will be a part of the device which is damaged by corrosion. If one area is protected better against corrosion than another area (e.g. by coating with a conventional coating process) the less protected area will be subject to corrosion instead. In other words, the corrosion problem “jumps” from one place to another, when the protection of a critical part is improved (relative to other parts). For example, if a corrosion problem on the battery contacts is fixed, then the battery casing itself may start to corrode.
The use of a ‘sacrificial anode’ is known from marine applications (e.g. outboard engines, underwater steel structures, ballast tanks on commercial ships, etc., cf. e.g. [Bardal; 1964], chapter 10.4, pp. 285-300) to prevent corrosion to attack e.g. steel parts exposed to salt water. A sacrificial anode is a metallic anode intended to be dissolved to protect other metallic components. The more (chemically) active metal (i.e. the metal having the least noble position in the activity series) is more easily oxidized than the protected metal and thus corrodes first (hence the term ‘sacrificial’).
U.S. Pat. No. 7,097,746 B1 describes an anode protection device in the form of a sacrificial anode plate located on a casing between positive and negative contact terminals of a device, e.g. a battery. In an embodiment, the sacrificial anodic plate is welded to the aluminium case of a rechargeable battery of a behind-the-ear (BTE) hearing device.